Heated backlights

ABSTRACT

A heated automobile backlight having a dark colored electroconductive grid. The composition includes silver powder, a glass frit such a lead borosilicate frit, and reducing agents such as stannous sulfate and chromic oxide.

This is a division of application Ser. No. 725,597 filed Apr. 22,1985now U.S. Pat. No. 4,623,389 to Danley et al, issued Nov. 18, 1986.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to heated windows, and more particularly,to heated windows for backlights in automobiles wherein the heatingelements are dark colored electroconductive circuits secured to asurface of the windows.

2. Technical Considerations

The deposit of moisture and ice on a automobile window has annoyedautomobile owners considerably. Automobiles that are parked overnightare dangerous to drive until visibility is attained by removing themoisture deposit. In the past, this removal has been accomplished byapplying a squeegee or scrapper to the surface of the window. Hot airblowers have also been employed to blow hot air across the surface of afogged window. However, time is required to heat the air that is blownacross the surface of the window to a temperature that is sufficient forthe hot air to perform efficiently in the defogging or deicing function.

In addition, the inner surface of automobile windows tends to fogwhenever the windows are closed and the moisture from the breath ofoccupants condenses on their inner surfaces. This source of fogging hasalso been difficult to remedy using the devices described above.

As an alternative to scraping, resistance wires have been attached tothe surface of monolithic glass sheets or laminated glass plasticwindows or embedded within a plastic interlayer of laminated windows toheat the window by passing electric current through the resistance wiresconnected between spaced bus bars. When a potential difference isapplied between the bus bars, the heating wires convert the electricenergy into sufficient heat energy to defog or deice the window asrequired.

It is common practice throughout the automotive industry to use silverceramic paste which adheres to the glass surface to form theelectroconductive heating circuits required to heat an automotivewindow. The pastes are generally silkscreened onto a glass sheet in apattern that generally includes thin parallel horizontal lines connectedat their ends near the edge of the window by wider bus bars. The silverceramic materials provide sufficient electrical resistivity in thethicknesses and widths at which they are applied such that current tothe electric circuit from an automobile 12 volt electrical energysystem, will cause the circuit to heat and thus defog or deice thewindow within a reasonable amount of time.

The color of typical production silver ceramic pastes after being firedonto clear float glass, when viewed through the glass, ranges from ayellow to a medium brown color depending on whether the paste is appliedto the air or tin side of a sheet of float glass. This color is slightlydarker when viewed through tinted float glass. It is believed that thiscolor results from the ionization of the silver of the paste duringheating, and little or no reduction of these silver ions. It is thedesire of automobile stylists, for cosmetic and aesthetic effects, tohave a darker grid line color for the electroconductive heatingelements.

Patents of Interest

U.S. Pat. No. 3,467,818 to Ballentine and U.S. Pat. No. 3,659,079 toWhittemore, teach an electrically heated window panel wherein the glasssurface silkscreen with thin lines of silver ceramic paste that areconnected by bus bars. The typical ceramic conductive coating materialincludes a highly conductive metal powder, such as silver, in avitrifying binder. The binder consists of lead borosilicate glass and acarrying medium.

U.S. Pat. No. 3,623,906 to Akeyoshi et al. teaches an electricallyheated rear window for a motor vehicle wherein a plurality of parallelstrips of an electroconductive frit are fired onto a glass window andelectrically connected together by bus bars. The frit consists of finelydivided electrically conductive metals such as silver, gold, copper orplatinum, a low temperature melting powdered glass, such as leadsilicate glass, and an organic binder.

U.S. Pat. No. 3,900,634 to Plumate et al. teaches an electrically heatedpanel with electrically conductive strips of paste having silverparticles intermixed with a liquid vehicle and glass particles having asoftening point lower than that of the glass substrate. The silver andglass particles are below five microns in size. The glass particlesinclude two different glass compositions.

SUMMARY OF THE INVENTION

The present invention provides an electrically heated glass product witha dark colored electroconductive coating. Colloidal silver encapsulatedby the glass immediately adjacent to the coating. The electroconductivecoating includes a lead borosilicate frit, silver powder and a silverion reducing agent. The silver ion reducing agent contains a trivalentchromium compound such as chromic oxide and a divalent tin compound suchas stannous sulfate.

The present invention further provides a method of applying anelectroconductive pattern to a glass sheet. A silver ceramic coatingincluding a silver powder, a glass frit, a silver ion reducing agent,and a carrying medium is applied to the glass to form a pattern. Theglass is heated to a temperature sufficient to ionize the silver powderat the surface of the glass and fuse the glass frit.

DETAILED DESCRIPTION OF THE INVENTION

The typical electroconductive circuit for a heated backlight includes aplurality of strips and bus bars made from an electroconductive fritcontaining paste. Typically, the strips are approximately 1/32" (0.079cm) wide and are interconnected at their ends by bus bars which areapproximately 1/4" (0.635 cm) wide. The electroconductive circuit isformed on the glass by conventional silkscreening methods, e.g. astaught in U.S. Pat. No. 4,433,623 to Beckim herein incorporated byreference. The thickness of the strips and bus bars is generally uniformdue to the nature of the silkscreening operation and ranges between 1 to1.5 mils.

Although not limiting to the present invention, when the backlight to becoated with such a design is rectangular or quadrilateral, havingsubstantially straight and parallel upper and lower longitudinal edges,the elongated electroconductive strips are generally spacedapproximately 1 inch (2.54 cm) apart and are parallel to one another andstraight. When the upper and lower edges of the backlight are bowed, orare of different configurations from one another, automotive stylistsgenerally prefer the electroconductive strips to extend between theopposed bus bars in arcuate paths, with the upper most strip conformingin curvature substantially to the curvature of the upper longitudinaledge of the backlight, and the lowest electroconductive coating has beenfound necessary.

After the desired pattern is applied to the glass, it is fired in afurnace where the glass is heated to its deformation temperature ofapproximately 1200° F. During this heating, the metal ceramic frit fusesonto the surface of the glass sheet, which faces upward during ahorizontal bending operation. The heat softened glass is then shaped,for example, by sandwiching the glass between a pair of press bendingmolds immediately outside the furnace or by some other shaping techniquewell known in the art. When the glass bending is completed, the glasssheet is removed from the hot atmosphere and chilled as rapidly aspossible to temper the glass sheet. Since the strips and bus bars havefused onto the glass surface during the heating operation, they remainin the exact configuration in which they were applied to the cold glassthrough the stencil in the silkscreening process when the glass ischilled.

After tempering, a means adapted for connecting the electroconductivecircuit to a power source is attached to the circuit. Although notlimited by this invention, generally this connection means is a metallicbraid or a terminal clip. Each is preferably soldered to the bus bar andprovides a connection to a lead wire and power source.

In accordance with this invention to provide a dark coloredelectroconductive circuit, the electroconductive strips and bus bars aretypically formed of an electroconductive coating material which includessilver powder, lead borosilicate frit, silver reducing agents, acarrying medium, and rheology control agents.

The purpose of each component of the composition and its proportionallimits, are defined as follows. The silver powder contributes to theelectrical conductivity of the circuit and provides the dark grid linecolor. The weight percent range for the silver powder is 60 to 85% witha preferred particle size range of 0.5 to 1.5 microns. The frit is usedto fuse and bind the silver ceramic coating to the glass surface. Thepreferred frit is a No. 2141 lead borosilicate glass frit available fromDrakenfeld Color Company, Pennsylvania. The weight percent range for thefrit is 2 to 15% with a preferred particle size range of 0.2 to 2.0microns. Other frits can be used as long as they are not reactive withthe silver and their melting point is not too high or too low withrespect to the temperatures used in a bending and tempering operation.If the melting point is too low, the silver powder particles willseparate and tend to move toward the upper surface of the coating. Ifthe melting point is too high, the frit will not melt and fuse thecoating to the glass surface. The reducing agents reduce silver ionsformed during the firing of the coating in a manner to be discussedlater. Chromic oxide and stannous sulfate are the preferred silverreducing agents with weight percentage ranges of 1 to 12% and 0.1 to20%, respectively, although other reducing agents with trivalentchromium and divalent tin can be used. These reducing agents can be usedindividually with varying effectiveness, but preferably a combination ofthe two should be used. A combination of chromic oxide and stannoussulfate preferably should not exceed 20% of the weight of the coatingsince the electrical properties and solderability of the bus bands willbe compromised. Pine oil is the preferred carrying medium and acts todisperse and mix the active components of the paste and to provideproper viscosity for the silkscreening process. The amount of pine oilis varied depending on the weight percentage of the other activecomponents. Although not limited by this invention, other compositionsthat can be used as a carrying medium include high molecular weightalcohols, such as cetyl alcohol and hexadecanol, and high molecularweight polyhydric alcohols. Rheology control agents are added to controlthe flow of the coating after it is applied to the glass. Although notlimited by the invention, isostearic acid, colloidal silica, and theamino salt are the preferred agents. Straight or branched chainhydrocarbon derivatives of a mono-basic carboxylic acid such asisovaleric acid, hexadecanoic acid, and isodecanoic acid can be used inplace of the isostearic acid. Morpholine fatty acid salts can be usedinstead of the amino salt. Aluminum oxide or zirconium oxide can besubstituted for the colloidal silica. The colloidal silica used in thepreferred embodiment is available under the tradename Cab-0-Sil fromCabot Corporation, Massachusetts. The amino salt is available under thetradename Bykanol-N from Mallinckrodt Inc., Missouri. As with the pineoil, the amount of these rheology control agents depends on the weightpercentage of the other active components.

When the silver ceramic coating containing the reductants is applied tothe air side of a sheet of float glass by silkscreening or in any othermanner well known in the art and fired to the heat deformationtemperature of the glass, the elemental silver in the coating near theglass surface ionizes and the silver ions diffuse into the hot glass.The chromic oxide and/or stannous sulfate reduce the silver ions thathave diffused a short distance into the glass substrate as well as thesilver ions at the glass surface, to elemental silver. The elementalsilver agglomerates to form silver colloids that are encapsulated by theglass. The encapsulated silver colloids are on the order of 100angstroms in diameter. The small particle size absorbs the lightwavelengths in the visible range and thus produces a darker color thanthe conventional paste when viewed through the glass, while theelemental silver remaining in the coating provides theelectroconductivity of the fired grid.

When the silver ceramic coating is applied to the tin side of a sheet offloat glass, the reducing agents again reduce the silver ions, but thereis additional silver reduction due to the stannous ions, Sn⁺², in thesurface of the tin side of the float glass. These stannous ions reducethe silver ions that diffuse deeper into the glass sheet. As a resultmore silver is reduced than on the coated air side and the large amountof encapsulated colloidal silver produces a dark or black grid color,when viewed through the glass.

It should be noted that when this composition is applied to the air sideof a sheet of float glass, there will be little if any Sn⁺² in thecoated surface to reduce sufficient additional silver ions to produce asdark a color as on the tin side coated glass. Increasing the amount ofstannous sulfate on the air side in an effort to increase the amount ofSn⁺² and thus increase the silver ion reduction would produce certaindeleterious effects described below.

The purpose of using both chromic oxide and stannous sulfate in thecomposition is to balance the reduction of the silver ions whileretaining conductivity in the electroconductive grid. The stannoussulfate will reduce some of the silver ions but a large amount of silverions must be reduced in order to produce a dark color in the grid. Ithas been found that if too much stannous sulfate is used, theconductivity and solderability of the bus bar will be reduced. To avoidthis detrimental effect, chromic oxide is added to help reduce therequired amount of silver ions to elemental silver without anydeleterious effect on the bus bar. If chromic oxide were used by itself,sufficient amounts of silver ions could be reduced to produce thedesired dark color but the solderability of the bus bar would besacrificed. In addition, the amount of chromic oxide must be carefullycontrolled so that a sufficient quantity of Ag⁺¹ is not reduced, so asto maintain the circuit's electroconductivity. If electroconductivityand/or solderability are of no concern, the chromic oxide could be usedwithout the stannous sulfate.

A series of tests were performed using varying amounts of thereductants, chromium oxide and stannous sulfate, to observe their effecton color, electrical resistancy, and solderability of a firedelectroconductive grid. During testing, the amount of oil was varied toachieve the proper viscosity during coating. As a result, the weightpercentage of silver powder varied between approximately 65% and 77%.

Testing for color required visual observation and comparison to thestandard production silver ceramic coating. In addition, sampleformulations were tested either for resistivity or for amperage, both ofwhich produce related results. The allowable amperage range is 18 to 22amps with 20 amps being the nominal result. To test solderability, apull test was performed. A metallic braid was soldered to the ceramicbus bar and weights were suspended from the braid at 90° from the glass.The acceptable pull test value range is 20 to 25 lbs. (9.09 to 11.36kg).

The following are examples of silver pastes as taught in the invention.

EXAMPLE 1

    ______________________________________                                                       Amount grams                                                                            Weight Percent                                       ______________________________________                                        Silver powder    37.62       68.40                                            Lead Borosilicate Frit                                                                         4.26        7.75                                             Pine Oil         7.02        12.76                                            Isostearic Acid  0.50        0.90                                             Amino Salt of an Acid                                                                          0.50        0.90                                             Phosphate Ester                                                               Colloidal Silica (SiO.sub.2)                                                                   0.25        0.45                                             Chromic Oxide (Cr.sub.2 O.sub.3)                                                               0.54        0.98                                             Stannous Sulfate (SnSO.sub.4)                                                                  4.30        7.82                                                              55.00       100.00                                           ______________________________________                                    

EXAMPLE 2

    ______________________________________                                                       Amount grams                                                                            Weight Percent                                       ______________________________________                                        Silver powder    42.00       76.36                                            Lead Borosilicate Frit                                                                         1.30        2.36                                             Pine Oil         6.00        10.91                                            Isostearic Acid  0.25        0.46                                             Amino Salt of an Acid                                                                          0.25        0.46                                             Phosphate Ester                                                               Colloidal Silica (SiO.sub.2)                                                                   0.20        0.36                                             Chromic Oxide (Cr.sub.2 O.sub.3)                                                               1.15        2.09                                             Stannous Sulfate (SnSO.sub.4)                                                                  3.85        7.00                                                              55.00       100.00                                           ______________________________________                                    

Example 1 has produced medium to dark red-brown electroconductive gridlines on the air side of float glass and Example 2 has produced darkgray and black electroconductive grid lines on the tin side of floatglass.

In testing the new silver ceramic compositions on the air side of floatglass three formulations were initially tested. The formulations werestandard production silver ceramic pastes except that one containedcopper selenide, one contained stannous sulfate, and the controlcontained neither. The fired bus bars did not develop the dark browncolor obtained in lab fired samples. Stannous sulfate turned out to bethe better additive, producing a yellow-brown color whereas the copperselenide gave the normal bright yellow color equal to the control. Thiscorresponded with the lab results which also showed stannous sulfate tobe better than copper selenide as a darkening additive. Other propertiesof the bus bars with the additives were acceptable. The electricalresistance of both was equal to the control. The adhesion to the glasswas good and equal to the control. Solderability checked out well andall three passed the pull test.

In the next set of air side tests, five different formulations weretested. The following amounts and combinations of chromic oxide andstannous sulfate as additives to produce a dark red-brown color on theair side were run: (1) 5% Cr₂ O₃, (2) 3.8% Cr₂ O₃ -1.2% SnSO₄, (4) 5%Cr₂ O₃ -5% SnSO₄ and (5) 8% SnSO₄ -2% Cr₂ O₃. The greatest coloration, amedium brown, was produced by the 5% CR₂ O₃ and 5% CR₂ O₃ -5% SnSO₄formulations. The resistances of the chromic oxide containingformulations were slightly higher than the production control, but theones containing stannous sulfate were equal to the control which was 75%silver. The five formulations ranged from 66% to 69% silver.Solderability of the bus bars having only the CR₂ O₃ additive was poor,while the formulation with 10% SnSO₄ was excellent. The other threeranged from good to fair. The 10% SnSO₄ samples had pull tests of 25-30lbs (11.36 to 16.64 kg) and those of the 5% Cr₂ O₃ -5% SnSO₄ sampleswere 12-25 lbs (5.54 to 11.36 kg). The rest were below these values. Theproduction control was in the range of 20-25 lbs (9.09 to 11.36 kg). Itis believed that the oxidizing conditions in the furnace nullified someof the reducing reaction of these two compounds which were found to bevery effective in the lab producing deep red-brown colors underlaboratory test conditions.

Additional testing on the air side continued with two formulations usingthe reducing agents stannous sulfate, SnSO₄, and chromic oxide, CR₂ O₃.Both formulations contained 15% by weight reducing agents - the first100% SnSO₄ and the other 3 to 1, SnSO₄ to CR₂ O₃. Both produced a mediumbrown color with the latter being slightly darker. Electricalresistances of the samples were equal to the production controlcontaining 67% Ag which was comparable to the 65% Ag in the testedformulations. Solderability and pull tests were good and equal to theproduction control for the bus bars produced from the formulation withthe 100% SnSO₄ additive, while the solderability and pull tests of the 3SnSO₄ /CR₂ O₃ additive was only fair. Again, it was observed that theoxidizing conditions prevalent in the furnace nullified some of thereducing reaction of these two compounds which were found to be veryeffective in laboratory testing.

The formulation shown in Example 1 was tested and produced a brown coloron the air side. Solderability was good with the pull test ranging from10-25 lb. (4.54 to 11.36 kg). The amperage test averaged 18.7 amps. Itis believed that the reason for the low amperage was that the circuitwas printed too thin causing the resistance to be a little high.

In the initial testing on the tin side of float glass, 15% stannoussulfate and 0% chromic oxide was used to coat glass. After heating andcooling the formulation produced a medium brown coating. Although theresulting color was darker than the standard silver ceramic paste whichis 75 weight percent silver and does not include any reductants, it wasstill not dark enough.

In the next set of tests on the tin side, two silver ceramicformulations were hand screened as bus bars in pairs across the width ofa backlight in order to compare properties within the same part. Thefirst formulation included 10% stannous sulfate and 5% chromic oxide.The second formulation included 10% stannous sulfate and 3% chromicoxide. Both pastes produced dark grid lines when compared to the lightyellow brown lines of the standard production paste. Subsequent testingof the fired coatings showed that the first formulation had an averagepull test of 15 pounds (6.82 kg) and had a 10% higher resistance whencompared to the standard coating. The second formulation had an averagepull test of 25 pounds (11.36 kg) and a resistance comparable to thestandard coating.

Tin side screened backlights using the 10% stannous sulfate and 3%chromic oxide formulation were produced for additional testing undersimulated production conditions. The resulting grid was again dark incolor when compared to the standard production coating. The circuittested slightly low at 17.6 amps but most of the pull tests wereacceptable in the range of 20 to 25 pounds (9.09 to 11.36 kg). However,two failures did occur.

Another silver ceramic formulation was tested with a 23% reduction ofreductants, while maintaining the same ratio of the two compounds. Theformulation included 2.3% chromic sulfate and 7.7% stannous sulfate. Arun of tin side screened backlights were compared with productionbacklights being run on the same line. Positive results were obtained asdark grid lines were formed, average amperage readings were 20 amps, andpull test values ranged from 20 to 33 pounds (9.09 to 15.0 kg).

To confirm the results of the prior test, another production type testtrial was conducted using a formulation of 2.3% chromic oxide and 7.7%dehydrated stannous sulfate. Dehydrated stannous sulfate was used todetermine if water content of the stannous sulfate effected the testresults. All properties were initially acceptable: dark colored gridlines, average of 20 amps for the amperage test, and pull test passed.However after running about 70 backlights, approximately one-half of thesolder joints failed and the amperage test dropped out of range to 17.5amps. The cause of this change in test results is not known.

The form of the invention shown and described represents an illustrativepreferred embodiment and certain modifications thereof. It is understoodthat various changes may be made without departing from the spirit ofthe invention as defined in the claimed subject matter that follows.

We claim:
 1. An electrically heated glass product including a glasssubstrate and an electroconductive coating on at least a portion of saidglass substrate, said coating comprising an electroconductive frit fixedto said glass and colloidal silver encapsulated within said glasssubstrate immediately adjacent to said frit.
 2. A heated glass productas in claim 1 wherein said electroconductive frit includes a leadborosilicate frit, silver powder, and a silver ion reducing agent.
 3. Aheated glass product as in claim 2 wherein said product is a heatedautomobile backlight and said coating includes a plurality of stripsoriented in generally parallel relationship and at least two bus bandsinterconnecting the ends of each of said strips.
 4. A heated glassproduct as in claim 3 further including means to provide electricalpower to said electroconductive frit.
 5. A heated glass product as inclaim 2, wherein said silver ion reducing agent contains a trivalentchromium compound and a divalent tin compound.
 6. A heated glass productas in claim 5, wherein said trivalent chromium compound is chromicoxide.
 7. A heated glass product as in claim 6, wherein said divalenttin compound is stannous sulfate.
 8. A heated glass product as in claim7, wherein said product is a heated automobile backlight, and saidcoating includes a plurality of strips oriented in generally parallelrelationship, and at least two bus bands interconnecting the ends havesaid strips.
 9. A heated glass product as in claim 5, wherein saiddivalent tin compound is stannous sulfate.
 10. The heated glass productas in claim 9, wherein said electroconductive coating is, by weight 1 to12% chromic oxide and 0.1 to 20% stannous sulfate, and further whereinthe total combined weight percentage of said chromic oxide and stannoussulfate is not more than 20%.
 11. An electrically heated glass productincluding a glass substrate and an electroconductive coating on at leasta portion of said glass substrate, said coating comprising anelectroconductive frit fixed to said glass and colloidal silverencapsulated by said glass immediately adjacent to said frit whereinsaid electroconductive frit includes a lead borosilicate frit, silverpowder, a trivalent chromium compound and a divalent tin compound.
 12. Aheated glass product as in claim 11, wherein said trivalent chromiumcompound is chromic oxide.
 13. A heated glass product as in claim 12,wherein said divalent tin compound is stannous sulfate.
 14. A heatedglass product as in claim 13, wherein said product is a heatedautomobile backlight, and said coating includes a plurality of stripsoriented in generally parallel relationship, and at least two bus bandsinterconnecting the ends have said strips.
 15. A heated glass product asin claim 11, wherein said divalent tin compound is stannous sulfate. 16.The heated glass product as in claim 15, wherein said electroconductivecoating is, by weight 1 to 12% chromic oxide to 0.1 to 20% stannoussulfate, and further wherein the total combined weight percentage ofsaid chromic oxide and stannous sulfate is not more than 20%.